Optics landing system and method therefor

ABSTRACT

A landing system is provided for accurate placing of collection optics in a microscope. In one example, a solid immersion lens (SIL) is used for light collection, and the landing system is operated to place the SIL in contact with an IC. A proximity sensor is used for determining the SIL&#39;s position with respect to the IC. The arrangement is attached to a z-motion stage. During the placement procedure, the navigation is performed in steps and at each step the compression of the SIL is measured relative to its uncompressed state. When a measured compression exceeds a preset threshold, a SIL landing is recognized. In one example, after a landing is recognized, a further compression is imparted to the SIL in order to place the SIL in a focusing distance to the objective lens.

RELATED APPLICATIONS

This application claims priority from, and is a Continuation in Part of,U.S. application Ser. No. 10/255,345, filed Sep. 26, 2002, now U.S. Pat.No. 6,828,811, which claims priority from Provisional Application Ser.No. 60/371,542, filed Apr. 10, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention and

The present invention relates to a system for accurate placement ofcollection optics in microscopes.

2. Description of the Related Art

Various microscopes are used in the art for imaging, testing, andexamination of various microstructures. A common feature of thesemicroscopes is that the obtained resolution depends on efficientcollection of light from the inspected object. Accurate placing of thecollection optics is also highly important for accurate focusing on theobject to be inspected. Additionally, to obtain efficient collection oflight, the collection optics needs to be accurately placed with respectto the object to be imaged.

While collection efficiency is highly important for many types ofmicroscopes, it is imperative in one particular field: probing andtesting of semiconductor microchips. Microchips need to be tested duringthe design and during the manufacturing stages. One type of testingrelies on light emission from the microchip that is generated whenever adevice, e.g., a transistor, on the microchip changes state. For furtherinformation on this phenomenon and it's investigation, the reader isdirected to, for example:

All-Solid-State Microscope-Based System for Picosecond Time-ResolvedPhotoluminescence Measurements on II–VI semiconductors, G. S. Buller etal., Rev. Sci. Instrum. pp. 2994, 63, (5), (1992);

Time-Resolved Photoluminescence Measurements in InGaAs/InPMultiple-Quantum-Well Structures at 1.3 μm Wavelengths by Use ofGermanium Single-Photon Avalanche Photodiodes, G. S. Buller et al.,Applied Optics, Vol 35 No. 6, (1996);

Analysis of Product Hot Electron Problems by Gated Emission Microscope,Khurana et al., IEEE/IRPS (1986);

Ultrafast Microchannel Plate Photomultiplier, H. Kume et al., Appl.Optics, Vol 27, No. 6, 15 (1988); and

Two-Dimentional Time-Resolved Imaging with 100-ps Resolution Using aResistive Anode Photomultiplier Tube, S. Charboneau, et al., Rev. Sci.Instrum. 63 (11), (1992).

Notably, Khurana et al., demonstrated that photoluminescence emissioncoincides with the switching of a transistor, thereby showing that, inaddition to failure analysis, the phenomenon can also be used for devicedebug and circuit design. See, also, U.S. Pat. No. 5,940,545 to Kash etal., disclosing a system for such an investigation.

As can be appreciated from the above-cited works, the light emission insemiconductor devices is very faint. Accordingly, various optical anddetection schemes have been proposed to more efficiently collect theemission and reduce the noise, i.e., increase collection fidelity,bandwidth, and speed. For example, commercially available microchannelphotomultipliers have been used to amplify the collected light byfactors of a million or so. Also, avalanche diodes coupled to time-to-amplitude converters (TAC) have been used to provide high temporalresolution of the emission.

From the optics perspective, various attempts have been made to increasethe collection of light and the resolution by increasing the numericalaperture (NA n*sinθ; n being the index of refraction and θ being thecollection angle) of the objective lens. It has been long known thatincreasing the numerical aperture can be achieved by increasing theindex of refraction, n, to be above that of air. One historical methodfor increasing n is to fill the gap between the objective lens and theobject with an index matching oil. Another method is to use an immersionlens between the object and the objective lens. Of course, one may useboth techniques, i.e., use immersion lens and index matching fluid. Theuse of the above techniques is disclosed in, for example, U.S. Pat. Nos.3,524,694, 3,711,186, and 3,912,378. More modem discussions of immersionlenses can be found in U.S. Pat. Nos. 4,634,234, 5,004,307, 5,208,648,5,282,088 and Solid Immersion Microscopy, S. M. Mansfield, G. L. ReportNo. 4949, Stanford University 1992. Prior art immersion lenses areplano-convex (i.e., hemispheres). That is, the bottom surface, i.e., thesurface facing the object, is flat, while the top surface, i.e., thesurface facing the objective lens, is convex.

A semiconductor device of particular interest to the present inventionis generally referred to as a “flip chip.” Since the structure of flipchips is known, it will not be discussed here in detail. Informationrelating to flip chips can be found in, for example,http://www.flipchip.com and http://world. std.com/˜hycomp/flippage.html.Of specific interest is recent effort in the art to inspect such devicesfrom the back side, i.e., from the substrate side. One problem intesting flip chips using conventional methods, such as e-beam testing,is that the metal lines are not readily accessible as in otherintegrated circuits. Therefore, in order to expose the metal lines tothe e-beam tester, one needs to employ a forced ion beam (FIB) to“drill” through the substrate and expose the metal lines. However, withthe density of today's IC's, the active devices occupy much of the “realestate” available on the substrate, thereby rendering the use of FIBimpossible. Therefore, the mechanism of light emission described abovehas been employed also for probing flip chips from the back side. Thereader is directed to these three articles, published in the Proceedingsof 1998 International Test Conference (ITC 98), Oct. 18–22, 1998,Washington, D.C., IEEE Catalog No. RSOO191:

Novel Optical Probing Technique for Flip Chip Packaged Microprocessors,Mario Paniccia, Travis Eiles, V. R. M. Rao and Wai Mun Yee.

Diagnosis and Characterization of Timing-Related Defects byTime-Dependent Light Emission, Dave Vallett, Leendert Huisman, and PhilNigh.

Contactless Gigahertz Testing, W. Mertin, A. Leyk, U. Behrike. and V.Wittpahl.

Another article of interest is Picosecond Noninvasive Optical Detectionof Internal Electrical Signals in Flip-Chip-Mounted Silicon IntegratedCircuits, H. K. Heinrich, IBM J. Res, Develop. Vol 34, No. 2/3 1990.

Systems for imaging flip-chips from the backside through the siliconsubstrate are described in U.S. Pat. Nos. 5,208,648, 5,220,403 and5,940,545.

However, in spite of the amount of work in the field, there is still nocommercially viable system for device debug by time resolvedmeasurements of hot electron emission, as opposed to deviceillumination. For example, one of the issues causing difficulties is theefficient collection of the faint light emission. Such an efficientcollection requires highly accurate control and placement of thecollection optics.

SUMMARY OF THE INVENTION

The present invention provides a landing system and method that enablesaccurate control and placement of the collection optics for amicroscope.

In one aspect of the invention, a landing system is provided for anintegrated system for testing an integrated circuit (IC). In thisparticular example, a solid immersion lens (SIL) is used for lightcollection, and the landing system is operated to place the SIL incontact with the IC.

In another aspect of the invention, the inventive system comprises aproximity sensor for determining the SIL's position with respect to themicroscope's objective. In one particular implementation, the proximitysensor is a physical contact sensor, e.g., a strain gauge or adifferential variable reluctance transformer (DVRT), attached to az-motion stage. This arrangement is coupled to an x-y stage that is usedto move the optics to the location of interest on the device under test.During the placement stage, the navigation is performed in steps and ateach step the compression of the SIL is measured relative to itsuncompressed state. When a measured compression exceeds a presetthreshold, a SIL landing is recognized. In one example, after a landingis recognized, a further compression is imparted to the SIL in order toplace the SIL in a focusing distance to the objective lens.

In yet another aspect of the invention, a SIL housing is movably mountedonto an objective lens housing. One part of a displacement sensor, e.g.,a strain gauge or a differential variable reluctance transformer (DVRT),is attached to the SIL housing, while the other part is attached to theobjective housing. This arrangement is coupled to an x-y-z stage that isused to move the optics to the location of interest on the device undertest. During the placement stage, the navigation is performed in stepsand at each step the motion of the SIL housing with respect to theobjective housing is measured. When a measured motion exceeds a presetthreshold, a SIL landing is recognized. In one example, after a landingis recognized, a further compression is imparted to the SIL in order toplace the SIL in a focusing distance to the objective lens.

In a further aspect of the invention, a SIL housing is movably mountedonto an objective lens housing. The SIL housing is spring-loaded againstthe objective housing so as to impart a non-linear resistive force tocompression of the SIL housing against the objective housing. A firstlinearly increasing force is imparted in a first compression range,defining a SIL landing range. Once the compression has surpassed thefirst compression range, indicating a SIL landing, a constant force isimparted over a second compression range, defining a focusing range. Thevariable source is provided by, for example, a non-linear spring, adual-spring arrangement and the like.

According to a particular feature of the invention, the landing systemis further provided with an interrupt to avoid damage to the SIL. Thesignal from the DVRT is fed to a Schmidt trigger that compares thesignal to a preset limit. When that limit is exceeded, an interruptsignal shuts down the stage to prevent damage to the SIL.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described herein with reference to particularembodiments thereof which are exemplified in the drawings. It should beunderstood, however, that the various embodiments depicted in thedrawings are only exemplary and may not limit the invention as definedin the appended claims.

FIGS. 1A–1B are general schematics depicting the major components of atesting and debug system which may be used with the landing systemaccording to embodiments of the invention.

FIG. 2 depicts an embodiment of the landing system according to anembodiment of the invention.

FIG. 3 depicts a block diagram of a landing system according to anembodiment of the invention.

FIG. 4 depicts a bi-convex solid immersion lens that can be used withthe landing system of the invention.

FIG. 5 depicts a differential variable reluctance transformer for use asa proximity sensor according to an embodiment of the invention.

FIG. 6 depicts an arrangement of collection optics and proximity sensoraccording to an embodiment of the present invention.

FIGS. 7 is a block diagram of a routine of a method according to anembodiment of the invention.

FIG. 8 depicts an arrangement of collection optics and proximity sensoraccording to an embodiment of the present invention.

FIG. 9 is a force vs. distance graph for a spring load mechanismaccording to an embodiment of the invention.

FIG. 10 depicts an arrangement of collection optics and proximity sensoraccording to another embodiment of the present invention.

FIG. 11 illustrates another embodiment using an optical arrangement forthe proximity sensor.

DETAILED DESCRIPTION

The present invention provides a landing system for a soft touchdown andaccurate placing of collection optics of microscopes. The system can beused with various arrangements of collection optics, and is especiallybeneficial for use in microscopes designed for detection of faint lightemissions. An example of such a microscope is depicted in FIGS. 1A and1B, illustrating an IC test and debug system. Of course, the inventivelanding system is suitable for use with other systems; however, forillustration purposes, various embodiments of the inventive landingsystem will be described with reference to the test and debug system ofFIGS. 1A and 1B.

FIGS. 1A and 1B are general schematics depicting the major components ofthe testing and debug system particularly suitable for timing and othertesting of microchips, especially from the backside through thesubstrate (e.g., flip chips). The system operates in conjunction with acommercially available automated testing equipment 105 (ATE). The ATEgenerally comprises a controller, such as a preprogrammed computer 181,and a test head 124 which comprises an adapter 125 used to deliversignals generated by the controller 181 to the device under test (DUT)110 in a manner well known in the art. Specifically, the ATE is used togenerate signals that stimulate the DUT to perform various tasks, asdesigned by the chip designer to check and/or debug the chip. Thevarious signals generated by the controller 181 are delivered to the DUTvia the adapter 125. The adapter 125 may include a space transformer, aDUT load board and a DUT socket, in a manner well known in the art.

In the embodiment depicted in FIGS. 1A and 1B, the ATE test head isplaced on top of a vibration isolated test bench 115, while the chamber100 that houses the entire optics, imaging and sensing system, and anX-Y-Z stage 175, is situated below. This provides a tremendous advantageas it allows the system to be used with any type and size of ATE withoutinterference with, or making modification to any of the elements insidechamber 100. Rather, the ATE is used to place the DUT from above, sothat it is visible to the optics 120 via opening 185. Stage 175 enablesplacing of the collecting optics at any locations within the opening185.

The various elements of the system of FIGS. 1A and 1B will be describedwith reference to its operational modes. Generally, the system operatesin two modes: navigation mode and detection mode. In the description,references to “navigation mode” should be understood to includenavigation, target acquisition, and imaging. Therefore, these terms maybe used interchangeably herein.

In the navigation mode, an illumination source 130 is used to illuminatethe DUT. Illumination source 130 emits light in the infrared rangeusing, for example, an IR laser, tungsten, or a halogen lamp. The lightis focused onto and then reflects from the DUT to be collected by thecollection optics 120 and selectively directed towards the imager 145via quarter wave plate 162 and dichroic mirror 160. The imager 145 canbe any two-dimensional detector capable of imaging in the infraredrange, such as, for example, a silicon intensifier vidicon. IR vidiconsare commercially available from, for example, Hamamatsu(http://usa.hamamatsu.com). Beam splitter mirror 165 is used to deflectpart of the collected light to the focusing system 140. Alternatively,the signal of the imager 145 may be used for focusing.

The collection optics 120 may be connected to the x-y-z stage 175.During placing of the collection optics 120, the z motion is controlledusing implementation of the present invention, as will be described inmore detailed below.

The switchable mirror 135 selectively varies the optical path betweenthe navigation and detection modes. For that purpose, mirror 135 may bemechanically moved or rotated, or it may be made of an active elementcapable of variable reflectivity according to the selected mode so that,for example, in one mode it is totally reflective, while in the othermode it is totally transmissive. For a rotational implementation, asubstrate may be made having half of it transmissive and the other halfreflective. Such a substrate can be provided with a rotating mechanismto insert in the optical path the reflective of transmissive part asdesired.

Additionally, a mechanized aperture 170 is provided at the image planeof the collection optics 120, together with field lens 195. Notably, inthis example, the image plane of collection optics 120 is generated intwo locations: at aperture 170 and at the detector 145. The mechanizedaperture 170 is illuminated from behind and is used to define the fieldof view at the image plane. That is, depending on the particular test tobe run, one may wish to select any particular section of the DUT foremission. Using information about the chip design and layout stored in aCAD software, such as, for example, Cadence, and using navigationsoftware, such as, for example, Merlin's Framework available fromKnights Technology (www.electroglass.com), one may select a particulardevice for emission test. When the user selects a device or location,the system activates the stage 175 so that the collection optics iscentered on the selected device or location. Then, the aperture 170 maybe adjusted to increase or decrease the field of view as appropriate forthe particular test desired.

When an appropriate field of view has been set and an image focused,mirror 135 is rotated so that the light path towards the JR sensitivedetector 150 is established. Additionally, light source 130 is shut offor blocked during testing. It should be appreciated, of course, thatchamber 100 prevents any exterior light from reaching any of the optics,imagers and sensors enclosed within.

Photon sensing during testing is done by detector 150, which may be aninfrared sensor, such as a photomultiplier tube (PMT), a photocathodecoupled to a multichannel plate (MCP), an avalanche photodiode (APD),etc. The signal from the detector 150 is sampled by the high-speed dataacquisition electronics 155. Controller 180, which may be ageneral-purpose computer running dedicated software, is used to controlthe various elements of the system, such as the stage and sampler. Thecontroller 180 receives sync signals from the ATE 105.

In order to maximize photon collection during testing, collection optics120 includes a solid immersion lens (SIL). The SIL may be such as theones disclosed in U.S. Pat. Nos. 5,004,307, 5,208,648 and 5,282,088, orany other suitable immersion lens. However, in this embodiment a novelbi-convex SIL is used. FIG. 4 depicts an embodiment of the bi-convex SIL400. Specifically, the upper surface 412 is convex in the form of ahemisphere with radial geometrical center at GC. The bottom surface 414is convex, but having a radius of curvature that is much larger thanthat of the upper surface. The radius of curvature of the bottomsurface, 414, is, for example, at least an order of magnitude largerthan that of the upper surface 412. The lowest point of the bottomsurface, 414, passes through the radial geometrical center, GC, of theupper surface.

In usage, the lowest point on the bottom surface may come in contactwith the object to be imaged 410, while the periphery of the bottomsurface at a few tens of nanometers therefrom. However, lens 400 canalso be used with a gap of up to about 200 nanometers from the object,wherein the gap is filled with either air or index matching material orfluid. In such a configuration, the periphery of the bottom surfacewould be a few tens of nanometers further from the object than thelowest point.

Since in the embodiment of FIG. 4 the bottom surface, 414, is convex, ithas a small and defined “point of contact” with the object. Note thateven if the lens does not contact the object, almost all of theradiation energy transmitted between the lens and the object would passthrough the lowest point of the convex bottom surface 414 of the lens,which is loosely defined herein as the “point of contact.” The small anddefined point of contact allows for accurate determination of the pointon the object that is being imaged. Also, by pressing the SIL againstthe DUT, the DUT may be deformed about the point of contact, therebycontrolling the surface contact of the SIL and the amount of lightcollected.

FIG. 2 depicts an embodiment of the landing system of the presentinvention. Collection optics 220 is used to collect light from DUT 210.In this embodiment, collection optics 220 includes a SIL 226 and anobjective lens inside housing 228. The collection optics is mounted to az-axis actuator 290 of an x-y stage 275. The motion of the stage andactuator is controlled by a computer 280, which may be an all purposecomputer programmed to perform specific tasks in accordance withembodiments on the invention.

A feature of this illustrative embodiment of the invention is that itcontrols the placement of the optics 220 with respect to the DUT 210.Advantageously, the depicted embodiment is capable of controlling a“soft” landing of the SIL 226 onto the DUT 210, and exerting apre-specified pressure onto DUT 210 by the SIL 226. That is, as can beunderstood, until the SIL is “coupled” to the DUT, no image can beobtained, so there's no way to determine where the SIL is with respectto the DUT. (The term “coupled” means to allow communication ofevanescent wave energy. In other words, the SIL is coupled to the DUTwhen it captures rays propagating in the DUT at angles higher than thecritical angle. As is known in the art, the coupling can be achieved by,for example, physical contact with the imaged object or very closeplacement of up to about 200 nanometers from-the object.) To that end, aphysical contact sensor, e.g., a compression sensor or a strain gauge230 is mounted onto the actuator 290 and/or housing 228 to serve as aproximity sensor so that the location of the STE with respect to the DUTcan be determined. That is, in this embodiment the compression sensor230 moves together with housing 228, so that its physical location isalways known with respect to the STE 226.

In the illustrative embodiment depicted in FIG. 2, the compressionsensor may be implemented in the form of, for example, a conventionalstrain gauge. Alternatively, a differential variable reluctancetransformer DVRT may be used. More specifically, a DVRT in the form ofan inverse solenoid may used. An example of a DVRT that can be used inthe embodiment of FIG. 2 is depicted in FIG. 5, showing a magnetic core534 that slides within a housing 532. The magnetic core 534 may bespring loaded by spring arrangement 536. The housing 532 houses aplurality of coils; in this embodiment one main coil 538 and twosecondary coils 537 and 539. In this embodiment, Va and Vb are connectedin series to secondary coils 537 and 539, respectively, but withopposite phase so that Va and Vb are subtracted from each other. Whencore 534 moves, the output voltage Vref changes and forms the outputsignal of the DVRT. Further information about DVRT and variouscircuitries can be found in the article entitled Accurate LinearMeasurement Using LVDTs by George Novacek, which can be found atwww.chipcenter.com/ernbapps/emba058.htm.

According to an embodiment of the invention, the compression sensor isused to precisely place the SIL in contact, and under pressure, with theDUT. A block diagram of an embodiment of the inventive landing systemthat can be used for that purpose is depicted in FIG. 3. A stagecontroller 300 controls the motion of the stage actuators and thelanding routine starts after the controller brings the optics 320 in x-yregistration with the location on the DUT sought to be examined. Then,the controller advances the z-motion actuator towards the DUT in astep-wise motion to provide a “soft” landing of the SIL onto the DUT.After each step, typically 10 μm to 100 μm increment, the compressionsignal from the DVRT 330 is sent to the computer 380 via interface 340and A/D card 350, either directly or through optional micro controller370. The computer 380 compares the signal to the signal obtained fromthe DVRT when it is in its uncompressed state. In this manner, thecomputer recognizes when the SIL contacts the DUT. In one embodiment,the computer instruct the stage controller to perform z-motion stepsuntil compression reading of the DVRT exceeds a preset value, typically4 μm to 10 μm, at which point landing is recognized.

As demonstrated in FIG. 3, a safety feature is incorporated into thisembodiment of the inventive system. Specifically, a hardware interruptis provided to prevent damage to the SIL in case the stage controllermoves the SIL beyond a set compression limit. In this example, this isdone using a limit switch 360, for example, a Schmidt trigger, thatreceives the DVRT signal from the interface 340. The switch 360 comparesthe signal to a preset signal, e.g., a set potentiometer voltage. Whenthe DVRT signal exceeds the preset signal, the stage is shut down via anexternal interrupt line that overrides the signal from the computer 380.

A further feature of the inventive system is depicted in FIG. 6. In FIG.6, the SIL 626 and an objective lens 654 are housed in collection opticshousing 628. The SIL 626 is attached to bracket 644 that is springloaded by springs 627. In this manner, the distance Df is variable andis used to perform focusing of the image. Specifically, once landing isdetected, the computer 380 calculates a distance Df required for startof focusing routine. The distance is calculated by knowing therelationship between the position of the core 634 of the DVRT 630 andthe SIL 626. That is, a calibration can be made beforehand, so thatmotion of the SIL in the Df direction is correlated with the output ofthe DVRT. The stage controller 300 then moves the optics 320 into startfocusing position, which may be a further compression of up to, forexample, 750 μm, but more typically 100–200 μm. In this example, thehardware limit is set to 750 μm, so if the computer sends a start offocus signal that requires a motion beyond 750 μm, the stage will beshut down by the switch 360. Of course, a check routine may additionallybe implemented in computer 380 to check whether the calculated motion isbeyond the switch limit and, if so, avoid sending the signal to thestage controller 300, but instead issue a warning to the user.

FIG. 7 is a flow chart depicting a method for SIL landing according toan embodiment of the invention. This method may be implemented as asubroutine performed by a general purpose or a dedicated computer. Thesubroutine may be implemented in software, hardware, or a combinationthereof. First, the routine initializes the stage (690, FIG. 6), readsthe (uncompressed) DVRT output, and sets the SIL compression to equalthe read DVRT output (steps 700, 710, and 720, respectively). Then, theroutine moves the stage to the starting point for the landing (Step730). In this embodiment, this starting point is a fixed z position ofthe collection optics with respect to the DUT (610, FIG. 6) to beinvestigated. This location may be entered manually by the user, orautomatically using a pre-programmed routine. This starting location maybe calculated to be, for example, 0.1–1.0 mm in front of the DUT, buttypically about 0.5 mm. This may depend on factors such as uncertaintyin the position of the DUT surface relative to the z-stage position, dueto, for example, DUT surface irregularities, DUT mounting errors, andSIL housing uncertainty.

Once the system has stabilized in its starting point, optionally thecomputer again reads the DVRT (Step 740) and sets the SIL compression tothe DVRT value (Step 750) in order to ensure accurate correlationbetween the SIL position and the DVRT compression. Then, the computersends a signal to the controller to move the z-stage one step, reads theDVRT, and sets the SIL current position to the DVRT compression. (steps760, 765 and 770, respectively). The routine then checks to see iflanding has occurred (Step 775). This can be done by comparing thecurrent DVRT reading to the initial or the previous one, or comparingthe current DVRT reading to a compression signal Vref, as explainedabove with respect to FIG. 5. If no landing has been detected, theroutine goes back to step 760 and performs another one step motion. If alanding has been detected, the routine calculates the SIL motionrequired to bring it to distance Df required for start of focusingoperation (Step 780). This distance can be calculated from the knownposition of the SIL, obtained by correlating the DVRT reading. Thecomputer then instructs the stage controller to move the stage in thez-direction the required amount so as to compress the SIL against thespring 627 until the appropriate distance Df has been achieved (Step785). Then, the routine is completed (Step 790) and a hand-off to thefocusing routine can be performed, or the user may be provided theoption to perform manual focusing.

An optional step in the embodiment of FIG. 7 relates to the decision oflanding. Specifically, in step 775, landing can be recognized only afterit has been determined that the SIL has been compressed against springs627 for a pre-determined distance. In one example, the distance is 4–10μm. Thus, until the DVRT measurement shows that the compression exceedsthis threshold, no landing is recognized and the routine goes back tostep 760.

Yet another embodiment is depicted in FIG. 8. In the embodiment of FIG.8, the SIL 826 is fixedly attached to the SIL housing 822, and theobjective lens 854 is fixedly attached to the objective housing 842 onbase 890. The SIL housing 822 can slide with respect to the objectivehousing, as exemplified by arrow S. In this manner, the distance Df canbe varied. To determine the position of the SIL with respect to theobjective lens, a proximity sensor is provided. While any proximitysensor may be used, two alternatives, 828 and 860, are depicted asexamples. In one example, a DVRT 830 is used, having its base 832attached to the objective housing and its movable core 834 attached tothe SIL housing. In this manner, any motion of the SIL housing 822 withrespect to the objective housing 842 necessarily causes a motion of thecore 834 with respect to the base 832 of the DVRT. Consequently, thereadout of the DVRT can be correlated to the distance Df.

Another example of a proximity sensor is a strain gauge arrangement 860.A lever 862 is fixedly attached to the SIL housing 822, and is urgedagainst a strain gauge 864. The strain gauge 864 is attached to theobjective housing 842. When the SIL housing 822 slides towards theobjective housing 842 the level 842 strains the strain gauge 864, sothat its signal can be correlated to the position of the SIL housing822. As can be understood, other proximity sensors may be used. Forexample, capacitive sensing system can be used to measure change inelectrical capacitance as the SIL housing moves with respect to theobjective housing. Similarly, optical system, such as, for example, aninterferometer or change in light intensity, can be used to measure theSIL housing's motion. Accordingly, the term proximity sensor is meant tocover any such measurement system.

A feature of the embodiment of FIG. 8 is the use of non-linear resistiveforce opposing the sliding of the SIL housing 822 towards the objectivehousing 842. This non-linear force can be provided by, for example, anon-linear spring, e.g., 856, or a combination of two linear springs,e.g., 856 and 852, having two different spring constants. What is soughtto be achieved is depicted in FIG. 9, which is a graph of force vs.sliding distance, Z, of the SIL housing. As shown in FIG. 9, as the SILhousing starts to slide towards the objective housing 842, it firstencounters a linearly increasing force which may be resulting from, forexample, a linear spring 856. Once the SIL objective reaches a certaindistance Z, the force is designed to be about constant. This can beachieved by the SIL housing urging against, for example, a second loadedspring 852.

In operation, the optics arrangement of FIG. 8 is advanced towards theDUT 810, either stepwise, as described above, or in continuous motion,until a landing is detected. The landing can be detected by a signal ofthe proximity sensor, for example, by sensing compression of the DVRT orthe strain gauge. The optics arrangement is then further advanced sothat the SIL is urged against the DUT with a force equal to the constantforce F1. As exemplified in FIG. 9, the SIL housing then may be moved inthe range Z1–Z2, which is the range where focusing is performed. Thus,using this embodiment, motion of the SIL to obtain various focusingdistances Df does not result in different pressures exerted on the SIL.

As noted above, the proximity sensor may also be implemented in the formof an optical sensor. Such an embodiment is illustrated in FIG. 10. Theembodiment of FIG. 10 uses a similar arrangement to the one illustratedin FIG. 8, inasmuch as SIL 1026 is fixedly attached to the SIL housing1022, and the objective lens 1054 is fixedly attached to the objectivehousing 1042. The SIL housing 1022 can slide with respect to theobjective housing 1042, as exemplified by arrow S, and may be springloaded by spring 1056. In this manner, the distance Df can be varied. Todetermine the position of the SIL with respect to the objective lens, aproximity sensor is provided. In this arrangement the proximity sensoris a light reflection arrangement 1030. The light reflection arrangement1030 is used to detect the motion of the SIL housing 1022 with respectto the objective housing 1042.

An embodiment of the proximity sensor in the form of a light reflectionarrangement 1030 will now be described with respect to FIG. 10. In thisembodiment, controller 1080 activates a light source 1010, e.g., an LEDor laser diode, to emit light. The emitted light is collected by fiberoptics 1012, having the output end thereof situated in a housing 1014,so as to emit a light beam LB. The light beam LB then reflects frommirror 1016. In this embodiment, the mirror 1016 is a polished “washer”that is physically affixed to the objective housing 1042. In thismanner, the housing 1014 may be placed anywhere around the SIL housing1022 and still illuminate the mirror 1016.

When the light beam LB hits the mirror 1016 it reflects back towards thehousing 1014 and is being collected by fiber optics 1018, the output endof which is exposed to light sensor 1020, e.g., PMT, MCP, APD, etc. Theoutput of the light sensor 1020 is provided to electronics 1024, whichprovides an intensity measurement of the received light. The intensitymeasurement is then provided to controller 1080. As can be understood,as distance Df becomes shorter, the light intensity collected by thefiber optics 1018 will increase. Thus, a correlation can experimentallybe derived to associate intensity level to various distances Df.

The embodiment of FIG. 10 may be used in a method for SIL landingaccording to an embodiment of the invention as will be explained herein.This method may be implemented as a subroutine performed by a generalpurpose or a dedicated computer. The subroutine may be implemented insoftware, hardware, or a combination thereof. First, the routineinitializes the stage, reads the reflected light intensity, and sets theSIL compression to equal the read intensity output. Then, the routinemoves the stage to the starting point for the landing. In thisembodiment, this starting point is a fixed z position of the collectionoptics with respect to the DUT to be investigated. This location may beentered manually by the user, or automatically using a pre-programmedroutine. This starting location may be calculated to be, for example,0.1–1.0 mm in front of the DUT, but typically about 0.5 mm. This maydepend on factors such as uncertainty in the position of the DUT surfacerelative to the z-stage position, due to, for example, DUT surfaceirregularities, DUT mounting errors, and SIL housing uncertainty.

Once the system has stabilized in its starting point, optionally thecomputer again reads the intensity output and sets the SIL compression(i.e., Df) to the intensity value in order to ensure accuratecorrelation between the SIL position and the intensity reading. Then,the computer sends a signal to the controller to move the z-stage onestep, reads the light intensity, and sets the SIL current position tothe intensity value. (that is, compression steps are correlated to lightintensity reading). The routine then checks to see if landing hasoccurred. This can be done by comparing the current intensity reading tothe initial or the previous one, or comparing the current intensityreading to a intensity signal Iref (Iref can be determinedexperimentally). If no landing has been detected, the routine goes backto and performs another one step motion. If a landing has been detected,the routine calculates the SIL motion required to bring it to distanceDf required for start of focusing operation. This distance can becalculated from the known position of the SIL, obtained by correlatingthe intensity reading. The computer then instructs the stage controllerto move the stage in the z-direction the required amount so as tocompress the SIL housing until the appropriate distance Df has beenachieved. Then, the routine is completed and a hand-off to the focusingroutine can be performed, or the user may be provided the option toperform manual focusing.

On the other hand, in embodiments where an index matching fluid is to beused, the focusing distance may be set as the “relaxed” position of theSIL housing 1022 with respect to objective housing 1042. Using theroutine described above, after a landing has been recognized thecomputer ma instruct the z-stage to move the arrangement away from theDUT for, say 100 μm. In this manner, space is provided between the SILand the DUT for the index matching fluid.

As can be understood, the features discussed above with respect tovarious embodiments can be easily employed in the embodiment of FIG. 10.For example, spring 1056 can provide linear or non-linear spring loadingand may be a single or multiple spring or other resilient meansarrangements. Similarly, a crush protection may be provided in variousforms. For example, the landing system may be provided with an interruptsignal to avoid damage to the SIL. According to one implementation, theintensity signal is fed to a Schmidt trigger that compares the signal toa preset limit. When that limit is exceeded, an interrupt signal shutsdown the stage to prevent damage to the SIL. Similarly, a lookup tablecan be constructed experimentally to define various alarm levels.

Another embodiment using an optical arrangement for the proximity sensoris illustrated in FIG. 11. The elements in FIG. 11 that are similar tothose in FIG. 10 have the same numerical references, except that theyare in the 1100's range. As can be seen, in this embodiment the lightfrom the light source 1110 is transmitted by fiber optics 1112 and ismade into a light beam LB that shines on the holder 1125 of the DUT1111. At least part of the holder 1125 is made of a reflective material,so that the beam LB is reflected by the holder 1125. Part of thereflected light is collected by the fiber optics 1118 and sent to theoptical sensor 1120, the output of which is sent to electronics 1124 todetermine the intensity of the reflected light. As in the embodiment ofFIG. 10, the measured reflected light intensity is correlated todistance: in this case the distance of the SIL 1126 to the DUT 1111.

As can be understood, the DUT 1111 is generally made of materials thatcan reflect light. Accordingly, the light beam LB can be made to shineon the DUT 1111, rather than on the holder 1125. As before, a lookuptable can be constructed experimentally to correlate the light intensityto distance of the SIL from the DUT.

While the invention has been described with reference to particularembodiments thereof, it is not limited to those embodiments.Specifically, various variations and modifications may be implemented bythose of ordinary skill in the art without departing from theinvention's spirit and scope, as defined by the appended claims. Forexample, while the proximity sensor used in the various examples is inthe form of a contact sensor, other techniques, such as, for example,optical interferometric techniques may be used. Additionally, all of theabove-cited prior art references are incorporated herein by reference.

1. A landing system for accurately placing collection optics to inspecta specimen, comprising: a controllable stage; a case situated on saidstage and housing collection optics therein; a proximity sensorproviding signals correlated to the proximity of said collection opticsto said specimen; a controller receiving the signals from said proximitysensor and providing output signal for controlling said stage so as toplace the collection optics at a specific location on said specimen; andwherein: said case comprises an objective lens housing; said opticscomprises an objective lens positioned inside said objective lenshousing and a solid immersion lens coupled to said objective lenshousing; and, said signals correlated to the proximity of saidcollection optics to said specimen comprise signals correlated to themotion of said solid immersion lens with respect to said objective lens.2. The system of claim 1, wherein said proximity sensor comprises anoptical sensor.
 3. The system of claim 1, wherein said proximity sensorcomprises light reflectance measuring system.
 4. A landing system foraccurately placing collection optics to inspect a specimen, comprising:a controllable stage; an objective lens housing affixed to the stage andenclosing therein an objective lens; a solid immersion lens housingslidably mounted on said objective lens housing and housing a solidimmersion lens therein a displacement sensor providing signalscorrelated to the motion of said solid immersion lens housing withrespect to said objective lens housing; a controller receiving thesignals from said displacement sensor and providing output signal forcontrolling said stage so as to place the solid immersion lens at aspecific location on said specimen.
 5. The system of claim 4, whereinsaid displacement sensor comprises an optical sensor.
 6. The system ofclaim 4 wherein said displacement sensor comprises a light reflectancemeasuring system.
 7. The system of claim 4, further comprising couplingmeans for imparting a non-linear resistive force to compression of saidimmersion lens housing against said objective lens housing.
 8. Thesystem of claim 4, wherein said signals comprise light intensitysignals.
 9. The system of claim 5, wherein said displacement sensorcomprises a light source providing illumination and wherein said opticalsensor sense reflectance of said light emission.
 10. The system of claim6, wherein said light reflectance measuring system comprises a lightsource providing illumination and an optical sensor sensing reflectanceof said light emission.
 11. The system of claim 9, further comprising afiber optics having an emission end affixed to one of said objectivelens housing and solid immersion lens housing, and a reflecting elementaffixed to the other one of said objective lens housing and solidimmersion lens housing.
 12. The system of claim 10, further comprising afiber optics having an emission end affixed to one of said objectivelens housing and solid immersion lens housing, and a reflecting elementaffixed to the other one of said objective lens housing and solidimmersion lens housing.
 13. The system of claim 5, further comprising aninterrupt mechanism.
 14. The system of claim 6, further comprising aninterrupt mechanism.
 15. A landing system for accurately placingcollection optics to inspect a specimen, comprising: a controllablestage; an objective lens housing affixed to the stage and enclosingtherein an objective lens; a solid immersion lens mounted onto saidobjective lens housing; a displacement sensor providing signalscorrelated to the motion of said solid immersion lens with respect tosaid objective lens housing.
 16. The system of claim 15, furthercomprising a controller receiving the signals from said displacementsensor and providing output signal for controlling said stage.
 17. Thesystem of claim 15, wherein said displacement sensor comprises anoptical sensor.
 18. The system of claim 15 wherein said displacementsensor comprises a light reflectance measuring system.
 19. The system ofclaim 15, further comprising coupling means for imparting a resistiveforce to compression of said immersion lens housing against saidobjective lens housing.
 20. The system of claim 19, wherein saidcoupling means imparts a non-linear resistive force.
 21. The system ofclaim 16, further comprising an interrupt mechanism.
 22. A collectionoptics arrangement comprising: an objective housing; an objective lensaffixed in the objective housing; a solid immersion lens movably mountedon said objective housing to thereby enabling change in the distancebetween the objective lens and the solid immersion lens; a sensorproviding a signal indicating change in the distance between theobjective lens and the solid immersion lens.
 23. The arrangement ofclaim 22, further comprising a resilient element imparting resilientforce resisting movement of the solid immersion lens.
 24. Thearrangement of claim 23, wherein said resilient element imparts anon-linear resilient force.